Cohabitating before your divorce is final should not have any direct impact on your child support calculation. Child support is calculated based on the income of each parent as well as the number of children and the needs of the children. Each state has its own specific mathematical formulas used to calculate the appropriate payment amount, and these payment formulas do not factor in whether or not you are living with someone else.
However, this is not to say that co-habitating can have no impact at all on child support. The impact it may have is an indirect one. If you are co-habitating with someone before the divorce is final, this could indicate to the court that your priority is not necessarily to create a stable home life for the child. This could, but will not always, affect the court's decision on custody. If you receive less visitation time with the child because the judge believes it is not in the best interests of the child to be a part of your new home that you are creating with your new partner, then your child supporting payments would go up because you would have less time with the child.
If you are concerned about the issues of child support and child custody, you should strongly consider scheduling a consultation with an attorney. A family law attorney can explain to you the formula for child support calculations in the state where you live and can help you to ensure you do not do anything to jeopardize your custody rights to your child or to otherwise adversely affect your rights during the divorce proceeding.